I’ve done a fair amount of googling and a look through the forum posts but can’t find anything that states whether or not it is possible to exit the Plex add-on then automatically close MrMC? I have found the option to launch the Plex add-on when MrMC starts but the family have got used to mashing the back button to get back to the FireTV dashboard, meaning MrMC never really exits and the Plex add-on isn’t there when MrMC is next “launched”.

What this does is is first, menu button will turn subtitles on/off. Second, back button will go back menus on one press instead of two. Third, when on the home screen of mrmc/Kodi the back button will exit the app.

So create a file called gen.xml in your keymap folder with this in it. Then in mrmc set it to launch to weather. Because weather doesn't exist it will launch straight to Plex addon. Now when you exit the Plex addon just mash back a few times and it will instantly exit and close mrmc.